The invention relates generally to the field of unidirectional electronic valves, and more particularly to a low voltage drop unidirectional electronic valve operating as a near ideal diode.
Solar power for large scale use, and/or for feeding into a power grid, is typically supplied by an array of serially connected solar panels. Each solar panel exhibits a positive terminal, and a return, or negative terminal. Solar panels generate electricity in the presence of an appropriate amount of sunlight, and thus one solar panel in the array may be in a dark condition, while others may be generating electricity. The dark condition may be caused by, among others, a flying object or bird, a cloud covering, or accumulated dirt. Electricity must be bypassed around the dark solar panel so that the output of the array is not blocked. Similarly, in the event of a failure of a single solar panel in the array, electricity must be bypassed around the failed solar panel so as to avoid failure of the entire array.
FIG. 1 illustrates an example of a technique known to the prior art to avoid failure of a solar array due to a dark or failed solar panel. The solar power arrangement of FIG. 1 comprises a plurality of solar panels 10, a plurality of bypass diodes 20, a blocking diode 30 and a converter 40. Solar panels 10 are connected serially, with the positive terminal of the ultimate solar panel 10 connected to the input of converter 40 via blocking diode 30. The return of converter 40 is connected to the return terminal of the first solar panel 10 of the arrangement. Each solar panel 10 has connected in parallel thereto a bypass diode 20, arranged to conduct only when the return terminal of the solar panel 10 to which it is connected exhibits a positive potential in relation to the positive terminal of that solar panel 10 in accordance with IEC 61215, published by the International Electrotechnical Commission, Geneva, Switzerland, and in particular section 10.18, the entire contents of IEC 61215 is incorporated herein by reference.
In operation, a dark solar panel 10 will exhibit a voltage reversal between the positive terminal and return terminal as a result of the current being driven into the return terminal from the positive terminal of the preceding solar panel 10. This voltage reversal rises to turn on the parallel connected bypass diode 20, thereby passing current around the dark solar panel 10.
The arrangement of FIG. 1 is successful in maintaining an output despite a dark solar panel; however the power dissipation of a bypass diode 20 is substantial. In a typical solar panel array, such as the arrangement of FIG. 1A, approximately 5-10 Amperes are flowing through each of the solar panels 10 in the array. Thus the power dissipation of a bypass diode 20, when operative as a bypass, is on the order of 3.5-7 Watts. The power lost to the system is emitted as heat, which thus drives thermal considerations for panel layout, construction of bypass diode 20 and ultimately cost of the arrangement of FIG. 1.
U.S. patent application Ser. No. 12/348,002 filed Jan. 1, 2009 entitled “Low Voltage Drop Unidirectional Electronic Valve”, the entire contents of which is incorporated herein by reference, is addressed to a low voltage drop unidirectional electronic valve comprising an electronically controlled switch constituted of a pair of field effect transistors, a control circuit and a refresh circuit arranged to periodically open the electronically controlled switch. The requirement for the periodic opening of the switch leads to a reverse voltage build up across the solar panel which is undesirable.
There is thus a long felt need for a low voltage drop unidirectional electronic valve adaptable for use as one of a solar panel bypass element not requiring a reverse voltage refresh.